Backscatter reduction apparatus

ABSTRACT

Apparatus for reducing F1/F1 repeater backscatter without significantly changing the desired received signal. A replica signal of the desired transmitted signal on center frequency F1 is transmitted on center frequency F2 to produce a received signal on frequency F2 which is predominantly due to backscatter from the transmission on F2 and which has a very high correlation with the backscatter from the desired transmitted signal on F1. The characteristics of the backscatter on F2 are accurately measured and then correlated to the backscatter on F1 whereby it can be reduced or minimized with relatively minor change of the desired received signal.

United States Patent [191 Lowry A Oct. 16, 1973 BACKSCATTER REDUCTION APPARATUS 75 l t z Primary Examiner-Albert J. Mayer 1 or Ray Lowry San Dlego Calif Attorney-R. S. Sciascia et al. [73] Assigneez The United States of America as represented by the Secretary of the 57] ABSTRACT Navy washmgton Apparatus for reducing F IF repeater backscatter [22] Filed: Sept. 8, 1972 without significantly changing the desired received signal. A replica signal of the desired transmitted signal [21] App! 287302 on center frequency F is transmitted on center frequency F to produce a received signal on frequency [52] US. Cl 325/65, 325/4, 325/8, F which is predominantly due to backscatter from the 325/18, 325/65, 325/67 transmission on F and which has a very high correla- [51] Int. Cl. H04b 7/14 ion with the backscatter from the desired transmitted [58] Field of Search 325/65, 42, l, 3, signal on F The characteristics of the baekscatteron 325/6, 12, 18, 67, 2, 8; 343/180 F are accurately measured and then correlated to the backscatter on F whereby it can be reduced or mini- [56] v I I References Cited I mized with relatively minor change of the .desired re- UNITED STATES PATENTS celved slgnal- 3,696,429 10/ l972 Tressa 325/12 7 Claims, 2 Drawing Figures r38 ,Is 18 A 5335 12 rRAm w T JEcE/vm AMPLIFIER l 20 I .1 FREQUENCY I as DOMAIN SUMMING FLTER CIRCUIT 54 22 l 7 I 'ggfijffl" MULTIPLEX TRAP INTEGRATOR mcurr I MIXER /32 I I INJECTION DELAY 26 I Damn I 55525535! I 28 l l BACKGROUND OF THE INVENTION It is well-known that backscatter from the ground or sea limits the gain of airborne or orbital repeater apparatus. Frequency offset techniques between the received and transmitter center frequencies have been used to reduce backscatter of both sampling and nonsampling repeaters. However, frequency offset techniques can only be used advantageously with signals having a very narrow bandwidth, and furthermore the techniques reduce allowable frequency difference between user transmitter and user receiver. Another tech nique for sampling repeaters comprise a selectively predetermined guard time between transmitter turn-off and receiver tum-on; however, this technique significantly reduces the energy of the desired received signal and the reduction in the energy of the desired received signal is approximately equal to the reduction of the energy of the backscatter signal.

SUMMARY OF THE INVENTION Apparatus are disclosed for attenuating backscatter from both sampled and non-sampled F /F repeaters whereby repeater gain can be increased substantially. The apparatus achieves the above without significantly changing the desired received signal on center frequency F and without requiring excessive receiver and transmitter frequency adjustments. The apparatus comprises means-for transmitting simultaneously on center frequency F a replica signal of the desired transmitted signal to produce on F a received signal which predominately consists of backscatter and which has a very high correlation with the backscatter on F The F backscatter further has characteristics which can be accurately ascertained whereby after correlation with F; backscatter, it can be readily attenuated.

STATEMENT OF THE OBJECTS OF THE INVENTION It is the primary object of the present invention to provide apparatus for reducing backscatter of F,/F, repeaters without significantly changing desired signals to thereby allow increased repeater gain to extend the range of communication systems.

It is a further object of the present invention to provide apparatus asdescribed above which can function BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical illustration of several exemplary frequency spectrums which are relevant to a complete understanding of the present novel backscatter reduction apparatus and techniques to be described hereinafter.

FIG. 2 is a simplified block diagram of apparatus embodying the present inventive concept.

DESCRIPTION OF THE PREFERRED I EMBODIMENT The essence of the novel inventive concept will initially be disclosed and described with respect to the frequency spectrums illustrated in FIG. 1 wherein waveform A is received by a repeater on center frequency F After a selectively predetermined delay period, the waveform B is transmitted on the same frequency F g or on a slightly offset center frequency F +A as shown by the waveform C.

the backscatter from the desired transmitted waveform B is received as either the waveform D-on the center frequency F or as the waveform E on the center frequency F,+A. The backscatter amplitude may be either smaller or larger than that of the waveform A.

The repeater also transmits a replica of the desired transmitted signal on center frequency F: as shown by waveform F and also receives the backscatter from the replica transmission on the same center frequency F, as shown by waveform G.

Frequency F +A may be either greater or smaller than frequency F,, i.e., the quantity A may be either positive or negative.'Likewis'e the frequency F may be either greater or smaller frequency F or frequency F +A. The replica signal (wavefonn F) may be either weaker or stronger than the waveforms B or C. 7

It can be appreciated that it would be extremely difficult to distinguish between the desired signalon center frequency F and the backscatter signal on center frequency F or F +A and to separate the two. Consequently it is very difficult and perhaps impossible to change in any manner either one of these two signals without also changing the other signal in some known or unknown manner.

The received signal on F includes a very small amount of the desired received signal on center frequency F the transmitted signal on centerfrequency F or F,+A, and the backscatter on center frequency F x or F +A. However, the received signal on center frequency F is predominately due to backscatter from the replica transmission on center frequency F The pertinent physical characteristics of the backscatter signal produced by the replica transmissionon center frequency F can be accurately ascertained by means of conventional frequency measurements, and- /or phase measurements, and/or relative measurements, and/or time measurements on the received signal on center frequency F 1 As stated earlier a high correlation exists between th backscatter signal from the transmission on center frequency F or F +A and the received signal on center frequency F since the latter is predominately due to the backscatter from the replica transmission on center frequency F and since the characteristics of the backscatter on center frequency F or F +A will be substantially identical to the characteristics-of the backscatter on center frequency F Minimal correlation exists between the desired received signal on center frequency F and'either of the two signals transmitted on center frequency F or F +A or on center frequency F This is so because a selectively predetermined delay period is introduced between the received signal and both transmitted signals. Therefore, minimal correlation or similarity exists between the desired received signal on center frequency F and either the backscatter signal on center frequency F or F +A or the received signal on center frequency F, since the two backscatter signals are generated by the two transmitted signals.

Consequently, after measuring its revelant characteristics, the backscatter signal from the replica transmission on center frequency F can be accurately correlated to the backscatter signal from the desired transmitted signal on center frequency F or F +A, whereby the backscatter signal on F can be attenuated (or increased) without significantly changing the desired received signal on center frequency F FIG. 2 is a simplified block diagram of a system which embodies the novel backscatter reduction concept described above. In FIG. 2, signals can be transmitted and received by means of a common antenna 10. If this is done, the transmit signals and the receive signals must be separated by apparatus such as a transmit/receive switch 16, although other devices such as filtered networks, isolation networks, or phase discriminators could also be utilized. Alternatively, signals can be transmitted by the antenna 12, and signals can be received on another antenna 14.

The received signals on F and those on F from either the antenna 14 or from the switch 16 are first amplified in the amplifier 18. Then the received signals on F are separated from the signals on F by the frequency domain filter 20. The filter 20 may include a digital filter to enhance its operation.

The separated signals on center frequency F are then coupled to a multiplexer integrator 22 which measures the signals to accurately ascertain the pertinent characteristics of the backscatter on center frequency F The separted signals on center frequency F are coupled to a frequency domain trap circuit which also receives the output from the multiplex integrator 22 and which functions to reduce the backscatter on center frequency F The attenuation of the backscatter on center frequency F is achieved with no significant change of the desired received signal on center frequency F The output of the trap circuit 24 comprises the received signals on center frequency less attenuated backscatter. This resultant output is delayed in the delay circuit 26 and then processed in the injection locked oscillator 28 to obtain isolation of the transmitted signal from the received signal. The oscillator 28 can also comprise a phase multiplexer, a time multiplexer, or a signal cancellation circuit. It can be appreciated that a portion of the isolation is also provided by the transmit/receive switch 16.

The replica signal on center frequency F is then added to the signal on F in the circuit 30 which comprises a mixer 32, and an offset frequency reference source 34. The mixer 32 generates the replica signal on center frequency F by mixing the signal from the offset frequency reference source with the transmitted signal on center frequency F obtained from the injection locked oscillator 28. The output of the mixer is fed to a summing circuit 36 which adds the replica signal on center frequency F to the transmitted signal from the oscillator 28 on center frequency F The resultant desired signal which comprises the output from the summing circuit 36 on center frequency F or F +A plus the replica on center frequency F is then amplified in the power amplifier 38. The amplified output is then transmitted on either the transmit antenna 12 or on the common receive/transmit antenna 10.

Thus it can be appreciated that novel apparatus and techniques have been disclosed for reducing backscatter in F /F 1 repeaters without significantly changing the desired signal and without decreasing the allowable frequency difference between the user transmitter and user receiver. Furthermore the inventive concept can be practiced without regard or concern for modulation type, bandwidth, and center frequency of desired signals.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. Apparatus for attenuating backscatter from F IF repeaters and comprising:

antenna means for receiving desired signals and corresponding backscatter on center frequency F and replica signals of said desired signals on center frequency F means for separating said desired signals and corresponding backscatter on F from said replica signals on F means for producing from said replica signal after said separation an output on F substantially equal to said backscatter on F means for subtracting said output on F from said desired signals and backscatter on F 1 to produce a resultant output substantially free from backscatter;

delay means for delaying said resultant output a selectively predetermined time period;

F /F repeater means for isolating desired-signals to be transmitted on F, from the delayed resultant sig- 'nal;

means for producing said replica signals on F means for adding said replica signals on F 2 to said desired signals to be transmitted on F from said FJF, repeater means to produce a desired resultant signal to be transmitted; and,

antenna means for transmitting said resultant signal to be transmitted.

2. The apparatus of claim 1 wherein said means for separating said desired signals and backscatter from said replica signals comprises a frequency domain filter circuit.

3. The apparatus of claim 1 wherein said means for producing said output on F comprises multiplexer integrator means.

4. The apparatus of claim 1 wherein said means for subtracting said output signal on F from said desired signals and backscatter on F 1 comprises a frequency domain trap circuit.

5. The apparatus of claim 1, wherein said F /F repeater means comprises an injection locked oscillator.

6. The apparatus of claim 1 wherein said means for producing said replica signals comprises an F, offset frequency reference source and a mixer means responsive to the output of said source and to the output of said FJF repeater means to produce said replica signals on F 7. The apparatus of claim 1 wherein said means for adding said signals comprises a summing circuit.

III i 

1. Apparatus for attenuating backscatter from F1/F1 repeaters and comprising: antenna means for receiving desired signals and corresponding backscatter on center frequency F1 and replica signals of said desired signals on center frequency F2; means for separating said desired signals and corresponding backscatter on F1 from said replica signals on F2; means for producing from said replica signal after said separation an output on F2 substantially equal to said backscatter on F1; means for subtracting said output on F2 from said desired signals and backscatter on F1 to produce a resultant output substantially free from backscatter; delay means for delaying said resultant output a selectively predetermined time period; F1/F1 repeater means for isolating desired signals to be transmitted on F1 from the delayed resultant signal; means for producing said replica signals on F2; means for adding said replica signals on F2 to said desired signals to be transmitted on F1 from said F1/F1 repeater means to produce a desired resultant signal to be transmitted; and, antenna means for transmitting said resultant signal to be transmitted.
 2. The apparatus of claim 1 wherein said means for separating said desired signals and backscatter from said replica signals comprises a frequency domain filter circuit.
 3. The apparatus of claim 1 wherein said means for producing said output on F2 comprises multiplexer integrator means.
 4. The apparatus of claim 1 wherein said means for subtracting said output signal on F2 from said desired signals and backscatter on F1 comprises a frequency domain trap circuit.
 5. The apparatus of claim 1 wherein said F1/F1 repeater means comprises an injection locked oscillator.
 6. The apparatus of claim 1 wherein said means for producing said replica signals comprises an F2 offset frequency reference source and a mixer means responsive to the output of said source and to the output of said F1/F1 repeater means to produce said replica signals on F2.
 7. The apparatus of claim 1 wherein said means for adding said signals comprises a summing circuit. 